The present invention generally relates to drilling fluids, and more particularly to biodegradable drilling fluids comprising organophilic clay treated with a quaternary ammonium surfactant having an amide linkage.
The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as oil or gas residing in a subterranean formation can be recovered by drilling wells that penetrate the formation. In particular, a well bore is typically drilled down to the formation while circulating a drilling fluid (also known as a drilling mud) through the well bore. During the drilling process, the drill bit generates drill cuttings that consist of small pieces of shale and rock. The drilling fluid carries the drill cuttings in a return flow stream back to the well drilling platform. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the well bore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the well bore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Subsequent secondary cementing operations, i.e., cementing operations occurring after the primary cementing operation, may also be performed.
Organophilic clays are commonly used in oil-based drilling fluids to increase viscosity and provide for the suspension of particles, such as drill cuttings and weighting agents, therein. Organophilic clays are generally prepared by reacting a hydrophilic clay with an organic cation, usually a quaternary ammonium salt compound produced from a fatty nitrile. Examples of hydrophilic clays include bentonite and hectorite. Native clay surfaces have negatively charged sites and cationic counter-ions such as sodium and calcium cations. Thus, the clay may be treated with a cationic surfactant to displace the cations that are naturally present at the clay surfaces. The cationic surfactant becomes tightly held to the surfaces through electrostatic charges. In this manner, the hydrophilic nature of the clay is reversed, making it more soluble in oil. By way of example, bentonite that primarily contains sodium cations is known as sodium bentonite. Those monovalent sodium cations may be easily displaced from the clay, making a large number of anionic sites available.
As mentioned above, quaternary ammonium compounds are commonly used as the cationic surfactants in preparing the organophilic clays. Quaternary ammonium compounds contain ammonium compounds in which one or more of the hydrogen atoms attached to the nitrogen are substituted by organic radicals. One of the most popular quaternary ammonium compounds of the organophilic clay industry is dimethyl dihydrogenated tallow ammonium chloride (M2HT2N+Cl−). The oil-solubility of this compound is enhanced by its almost complete hydrocarbon structure and its two long-chain alkyl groups. Further, its two methyl groups do not sterically interfere with close packing of the ammonium cation to the clay surface.
The dimethyl dihydrogenated tallow ammonium chloride surfactant, however, cannot be efficiently activated at relatively low temperatures. Improved cationic surfactants have been developed in which the ammonium compounds have greater numbers of alkyl groups. One such surfactant includes a benzyl group that greatly enhances the performance of organophilic clays at cold temperatures near 45° F.
Unfortunately, the quaternary ammonium compounds described above undergo minimal aerobic biodegradation and are toxic and thus may harm the environment. Over time, those compounds may accumulate and become toxic to life, thereby posing a much greater risk to the environment. Due to environmental regulations, costly procedures often must be followed to dispose of drilling fluids containing organophilic clays treated with such compounds, ensuring that they do not contact the marine environment and groundwater. Thus, it is desirable to develop a biodegradable cationic surfactant for use in the formation of an organophilic clay additive for a drilling fluid.